In nuclear reactors, especially pressurized water and boiling water reactors, the reactor core is located inside a closed chamber which constitutes a tank in which fluid which is intended to cool the core and supply energy is heated. This fluid is exhausted and passed to steam generators through pipes the ends of which are attached to pipe stubs on the tank. The same goes for the pipes which bring the cooled fluid back into the tank. In view of the pressures to be withstood, the tank normally consists of superimposed collars of very thick plate. The collars may be obtained by forging from a single annular piece or by welding a number of "plastrons" of curved plate each constituting a segment of a cylinder.
In the same way pipe stubs must be provided on the steam generator and especially on the ends of these generators which often are shaped as a segment of a sphere.
The ends and the collar furnished with pipe stubs are manufactured in two main ways.
Firstly, one can make a casting in one piece. It is then sufficient to provide a pipe stub in the mould. The casting, however, is objected to by users for various reasons and its dimensions are always limited.
An end or collar may likewise be produced in the form of stamped pieces of plate in which apertures have been provided, the diameter of which is equal to the maximum diameter of the pipe stub. The pipe stub then consists of special cast or forged pieces which are welded on to the plate along the edge of the aperture.
The machining of the apertures, production of welds in very thick walls under difficult conditions, especially when the weld must be curved, and the numerous checks necessary increase the cost of manufacture very substantially. On the other hand, for safety reasons it is necessary to check regularly the hold of the welds, which necessitates stopping the operation of the plant including a chamber produced in this way.